The US government's only facility for handling, processing and storing weapons-grade uranium has been temporarily closed after anti-nuclear activists breached security fences.

The facility at Oak Ridge, Tennessee, was shut down on Wednesday after at least three protesters, including an 82-year-old nun, cut through perimeter fences to reach the outer wall of a building where highly enriched uranium, a key nuclear bomb component, is stored.

WSI Oak Ridge, the private contractor responsible for protecting the facility, is a subsidiary of G4S, which has faced heavy criticism for its failure to provide sufficient staff at the Olympic Games in London.

The activists, who called themselves Transform Now Plowshares, painted slogans and threw what they said was human blood on the wall of the facility, according to government officials.

Although the activists triggered security sensors they were still able to reach the building's walls before WSI Oak Ridge staff intercepted them.

In a statement the three activists said they had passed through four fences and walked for "over two hours" before reaching the uranium storage building, on which they hung banners and crime-scene tape.

The activists' spokeswoman, Ellen Barfield, said three were arrested and charged with vandalism and criminal trespass.

She said the three, identified as Megan Rice, 82, Michael Walli, 63, and Greg Boertje-Obed, 57, were being held in custody and appeared for a hearing on Thursday before a US magistrate judge in Knoxville, Tennessee.

A detention hearing is set for Friday afternoon, when prosecutors must show the defendants are a flight risk and a danger to the community in order to keep them in custody, according to court officials. The trial date is 9 October.

A spokeswoman for G4S declined to comment.

Government officials said the contents of the facility were not compromised.